1. Field of the Invention
The invention relates to a dielectric antenna having at least one supply element and at least one lens consisting of at least one dielectric material, wherein the supply element emits electromagnetic radiation and supplies the lens with the electromagnetic radiation, wherein the lens guides the electromagnetic radiation further and radiates it.
2. Description of Related Art
In industrial measurement technology, radar fill level sensors are often used for determining the fill level of media, such as liquids, bulk materials and also slurries in containers, such as tanks or silos. The transit time method implemented by the measuring device is based on the physical law that the path, e.g., of an electromagnetic signal is equal to the product of transit time and propagation speed. In the case of measurement of the fill level of a medium—for example, a liquid or a bulk material—in a container, the path corresponds to twice the distance between an antenna emitting an electromagnetic signal and receiving it again and the surface of the medium. The wanted echo signal—i.e., the signal reflected on the surface of the medium—and its transit time are determined using the so-called echo function or the digitized envelope curve. The envelope curve represents the amplitudes of the echo signals as a function of the distance “antenna—surface of the medium”. The fill level can be calculated from the difference between the known distance of the antenna to the floor of the container and the distance of the surface of the medium to the antenna determined by measurement. These emitted and received electromagnetic signals are usually microwave radiation.
Dielectric resonators are often used as antennae. Such dielectric antennae have a resonance behavior similar to hollow waveguides, but can, since they do not have metal walls, radiate electromagnetic radiation, and thus, function as antennae. As a result, several models are known from different areas of technology in each of which a dielectric material, e.g., ceramics, aids in guiding and radiating the electromagnetic waves. Usually, low-loss materials with low permittivity (another term for permittivity is dielectric conductivity) are used. Polytetrafluoroethylene or polypropylene is used, for example.
In particular, in the case of bulk materials, difficulties can arise in determining the fill level due to the occurrence of material cones. Objects, such as, e.g., a stirring device, can be further found in the container in any type of medium, these objects reflect the radar waves and cause disturbance in measuring the fill level. For this reason, it is advantageous, in some applications, to not only measure a region of the surface of the medium, but also to determine the distribution of the medium, i.e., the surface contour. Consequently, in some applications, each distance to the antenna is determined for several measuring points or measuring regions that are correspondingly distributed over the surface of the medium.
In order to measure such a surface contour, it is of particular advantage when the antennae used have a narrow as possible main direction of radiation—i.e., a narrow directivity in the main direction of radiation. Narrow directivity requires mostly a large aperture—i.e., opening area—of the radiating section. So that the aperture is also used in the sense of a narrow main direction of radiation, the electromagnetic radiation radiated from the radiating section of the antenna has to have a phase front that is as even as possible. In horn antennae known from the prior art, a narrow directivity goes hand in hand with large geometric dimensions of the antenna. In industrial measurement technology, however, small and compact measuring instruments are mostly preferred. For this reason, dielectric lenses are also used in the prior art, consisting mostly of a mixture of a resin—for example, polypropylene, polyethylene or polystyrene—and a ceramic powder. The effect, for example, of convex delay lenses is that the lens delays the radiation close to the axis as opposed to the radiation at the edges. The refracted radiation beams are parallel and in phase after passing through the lens.
European Patent Application EP 1 701 142 A2 and corresponding U.S. Pat. No. 7,408,501 B2 describe an arrangement for measuring a surface distribution of a medium using the radar principle, wherein several emitting antennae and several receiving antennae are used. These emitting and receiving antennae are individually combined with one another in order to measure more regions of the surface of the medium. Due to the number of antennae, construction is very complex.
Details for designing a dielectric antenna that has a hollow waveguide as supply element with a lens-like radiating section are found, for example, in German Patent Application DE 10 2008 020 036 A1 and corresponding U.S. Patent Application Publication No. 2009/0262038 A1.
German Patent Application DE 10 2008 008 715 A1 and corresponding to U.S. Patent Application Publication No. 2010/0321262 A1 shows an ellipsoidal lens antenna, at least in the direction of radiation. Spherical waves are converted into waves having parallel phase fronts due to the shape of the antenna. If the distance between the supply element and the antenna is changed, the direction of radiation is also changed with it. However, this is disadvantageous in that the radiation patterns are impaired by the shifting of the direction of radiation. This goes hand in hand with a reduction of the resolution at different positions of the object to be measured, i.e., the surface of the medium in measured containers.
U.S. Patent Application Publication 2003/0006941 A1 and corresponding U.S. Pat. No. 6,606,077 disclose a circular lens for a radar measuring device. Several supply elements are provided with it, each being individually controlled in order to directly change the direction of radiation.
German Patent Application DE 38 40 451 A1 describes a lens antenna having a supply element. The lens profile is designed in such a manner that spherical waves are changed in some planes into waves with even phase fronts and in other planes into waves with uneven phase fronts. In order to adjust the planes of radiation, it is provided to turn the supply element. Furthermore, the distance between supply element and antenna can be changed for calibration purposes.
A device for calibrating a radar antenna is described in German Patent Application DE 602 03 320 T2 and corresponding U.S. Pat. No. 6,531,989 B1. A lens is provided that has a concave side and a convex side.
Several antennae are known from the prior art with which the surface measurement of media can be implemented. Mostly, problems include the large amount of effort or space requirements or non-consistent measurement accuracy in all measured spatial areas.